Precipitation behavior and chemical composition of secondary γ‛ precipitates in a Re-containing Ni-based single crystal superalloy

Three-dimensional atom-probe tomography and scanning electron microscopy were utilized for an in-depth examination of the formation of secondary γ′ precipitates and the composition difference between secondary γ′ precipitates and primary γ′ precipitates in a Re-containing Ni-based single crystal superalloy. It was found that the secondary γ′ precipitates was formed during the air cooling after primary aging of 1180 °C/4h and would grow up during the secondary aging at 870 °C. Al and Ta were mainly distributed in γ′ precipitates (including the primary and the secondary γ′ precipitates) and Mo, Cr, Co and Re were more distributed in γ matrix. Compared with the primary γ′ precipitates, the secondary γ′ precipitates contained more γ-forming elements, such as Re, Cr, W and Mo, while the content of Ta was less. The distribution of W in the secondary γ′ precipitates was much higher than that in the primary γ′ precipitates and was equal to that in γ matrix. Based on the diffusion constant of alloying elements in Ni, the diffusion distance of various elements was as followed during the air cooling after primary aging: Re < W < Co < Mo < Ni < Ta < Cr < Al. The insufficient diffusion distance of γ-forming elements was an important factor leading to their enrichment in the secondary γ′ precipitates. And the Re element might hinder the diffusion of other γ-forming elements, such as Cr and Mo.